Exploring the Dietary Preferences of Limnic Supraglacial Consumers: ¹³ C and ¹⁵ N Isotopic Food Labelling in Tardigrades and Rotifers From Cryoconite Holes

Cryoconite holes are among the most biologically active and diverse limnic habitats on glacier surfaces. The apex cryoconite consumers, tardigrades and rotifers, are known to contribute significantly to the processing and composition of supraglacial (i.e., glacier surface) organic matter (OM). Yet, their specific food sources remain unknown. We experimentally added two contrasting isotopically labelled food sources, 13 C‐labelled algae and 15 N‐labelled bacteria, in four treatment setups to cryoconite holes in situ. After incubations, tardigrades and rotifers were collected and analysed for their carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopic composition. δ 13 C and δ 15 N values of consumers from non‐labelled control samples were compared to those previously measured from the same glaciers, with seasonal variability in isotopic signatures found in some samples. Both consumers select their food and likely feed on both algae and bacteria, albeit the consumption efficiency of filter‐feeding rotifers was generally higher. Tardigrades showed a significant but low 13 C enrichment compared to control samples in treatments with 13 C‐labelled algae, while in treatments with 15 N‐labelled bacteria, both consumers showed comparably high enrichment in their δ 15 N. This study presents the first empirical study on food preferences for algae and bacteria in supraglacial tardigrades and rotifers and differences in their consumption rates, significantly deepening our knowledge of the consumers' role in supraglacial OM composition and nutrient flow through the cryoconite food web.